1. Field of the Invention
The present invention relates to a wastewater treatment process for treating wastewater containing a solid fraction that is discharged during a process for producing electrophotographic toner (electrostatic latent image developing toner) or the like.
2. Description of the Related Art
At production facilities for water-based paints and electrophotographic toners and the like, processes for the production of water-based dispersions generate wastewater that contains dispersed colorants and the like. Because this wastewater contains not only a colorant component containing pigments and the like, but also surfactants that have been used for improving the dispersibility of the pigments, and silica that has been used as an internal additive, both the solid fraction concentration and the chemical oxygen demand (COD) are large, meaning the wastewater is unable to be discharged into rivers or ground water in an untreated state. As a result, this wastewater should first undergo treatment within a wastewater treatment facility inside the production plant, prior to being discharged externally.
In recent years, a variety of chemical toner production methods, including emulsion polymerization methods, suspension polymerization methods, and solution suspension methods have been developed and implemented to replace the more conventional mix-and-grind electrophotographic toner production methods. For example, in an emulsion polymerization method, a resin dispersion formed by emulsion polymerization of a binder resin polymerizable monomer, together with a colorant, a release agent, and if required a charge control agent and the like, are stirred together in a water-based solvent in the presence of a surfactant, and subjected to aggregation, heating, and fusing, thereby preparing toner particles comprising colored resin particles with a predetermined particle size, particle size distribution, shape, and structure. However, during this process, wastewater is generated that contains a significant solid fraction, including aqueous solutions of the surfactant, pigment dispersions, release agent dispersions, aqueous emulsions, silica dispersions, and wastewater from apparatus cleaning operations.
Normally, coagulation-sedimentation treatment is used as the most typical wastewater treatment process. As disclosed on pages 141 to 153 of the 5th edition of “Technology and Regulations for Pollution Prevention (Water Resources Edition)” (edited by the Environmental Protection and Industrial Location Bureau, Ministry of International Trade and Industry, published 2001), coagulation-sedimentation treatment is the most widely used solid-liquid separation operation within the field of wastewater treatment. In a coagulation-sedimentation treatment, a coagulant is added to the wastewater to generate a flock (large, coarse particles generated by aggregation), and the difference in specific gravity between the water and the flock causes the flock to sink as a sediment, thereby achieving a solid-liquid separation. The solid flock that is separated in this manner is treated as an industrial waste sludge, whereas the remaining water, which exhibits a significantly reduced chemical oxygen demand, is discharged into rivers or ground water. The sludge generated by the solid-liquid separation is often subjected to dewatering using a pressure filtration dewatering apparatus. As described on page 182 of the 5th edition of “Technology and Regulations for Pollution Prevention (Water Resources Edition)” (edited by the Environmental Protection and Industrial Location Bureau, Ministry of International Trade and Industry, published 2001), a pressure filtration dewatering apparatus is the most commonly used dewatering system in the field of wastewater treatment.
Japanese Laid-Open Publication No. Hei 7-136408 discloses a wastewater treatment process for conducting a solid-liquid separation of wastewater containing organic contaminants, wherein final treatment of the sludge is simplified by recycling a portion of the generated sludge and returning it to the raw treatment water. Furthermore, Japanese Laid-Open Publication No. Hei 9-225474 discloses a process wherein a pressure flotation separation is used for treating an emulsion wastewater.
However, the solid fraction and chemical oxygen demand of the wastewater generated at an actual toner production plant can vary considerably depending on the nature of the process, and if the process disclosed in Japanese Laid-Open Publication No. Hei 7-136408 is used for treating wastewater with a low solid fraction concentration or low chemical oxygen demand, then because the quantity of the flock formed by addition of the coagulant is minimal, the sludge does not settle readily, meaning the solid fraction concentration or the chemical oxygen demand does not decrease significantly. In those cases where this process is used for treating silica-containing wastewater, the action of the silica causes the sludge to become particularly resistant to sedimentation, although the mechanism for this action is not clear.
Furthermore, although the process disclosed in Japanese Laid-Open Publication No. Hei 9-225474 is suitable for the treatment of emulsion wastewater, it cannot be applied to all the various types of wastewater that are generated in a toner production plant.
Furthermore, the wastewater generated during electrophotographic toner production using a chemical toner production method such as an emulsion polymerization method, suspension polymerization method, or solution suspension method contains surfactants, colorants, release agents, silica, and a variety of other components, and if this wastewater is treated using a coagulation-sedimentation treatment, then the settling characteristics of the flock within the settling tank are poor, and particularly in those cases where the solid fraction concentration or the chemical oxygen demand is low, an extremely long settling time is required to achieve satisfactory solid-liquid separation. In addition, if a large quantity of coagulant is used to effect a satisfactory coagulation-sedimentation treatment of the small quantity of solid matter contained within the wastewater, then the quantity of sludge that should be treated as industrial waste increases significantly.
The present invention is a wastewater treatment process which enables efficient treatment of wastewater containing a solid fraction that is discharged during processes for producing water-based dispersions at production facilities for water-based paints and electrophotographic toners and the like, and also enables reductions in the quantity of coagulant used and the quantity of sludge generated.